The present invention relates to a method for forming register marks which are used to attain automatic registration between a plurality of printing plates on an offset printing press.
Heretofore, various methods for making automatic corrections of misregistration by reading register marks formed or marked on printing plates or printed sheets in the case of printing by an offset printing press have been proposed, but the problem of maintaining a correct positional relationship between a register mark and a pattern or image to be printed has not yet been solved. As a result, automatic correction of misregistration or misalignment has been practically impossible.
For instance, a method in which in addition to a film original for printing a pattern or image, a film original bearing a register mark is provided, and, prior to or after the pattern or image is printed, the film original bearing a register mark is aligned with the film original bearing a pattern or image by a pin-and-hole registration or alignment method has been proposed and demonstrated. This method has a serious drawback in that a register mark and a pattern or image to be printed are not correctly registered or aligned with each other.
Another method in which, prior to or after a pattern or image is printed, a film original bearing a register mark is set on a printing plate in a composer (step and repeat machine) so that a register mark is printed or otherwise formed on a printing plate in a manner substantially similar to that in which a pattern or image is printed has also been proposed. This method is advantageous over the first-mentioned method in that a register mark can be printed or otherwise formed at a specific position with a high degree of accuracy, but it has a disadvantage in that since register marks must be printed or otherwise formed on both sides of a printing plate, additional printing operation for printing two register marks is necessary.
In general, the composer prints four to eight patterns or images on a single printing plate so that the efficiency of the composer is adversely affected when two additional register marks are printed or otherwise formed.